Experimental Cosmic Microwave Background

Draft version February 15, 2010 Preprint typeset using LATEX style emulateapj v. 08/13/06 EXPERIMENTAL COSMIC MICROWAVE BACKGROUND RESEARCH IN CANADA WHITEPAPER FOR THE CANADIAN DECADAL LRP Matt Dobbs1, Dick Bond2,3, Mark Halpern4, Gil Holder1, Barth Netterfield2, Douglas Scott4 Draft version February 15, 2010 ABSTRACT The cosmic microwave background (CMB) is a cornerstone for precision cosmology and Canadian experimeters have been involved with the majority of significant results over the last decade. In this whitepaper, highlights from this period are presented along with a summary of our unique capabilities. With the temperature anisotropies accurately characterized out to moderate angular scales (` < 700) the community is focused on (1) small angular scale temperature anisotropy measurements to study the intervening matter through CMB secondaries and (2) the CMB polarization onto which the signature of inflationary gravity waves may be imprinted, allowing us to probe back to a fraction of a second after the big bang. In parallel with this, CMB experimenters are also planning new projects at other wavelengths, continuing the already established tradition (e.g. BLAST and SCUBA2) of using expertise and technology originally developed for mm-wave CMB observations to break new ground elsewhere. Subject headings: Cosmic Microwave Background, Cosmology, Canada 1. INTRODUCTION At small angular scales there is a wealth of information Measurements of the Cosmic Microwave Background about inflation and the growth of cosmic structure. The radiation have provided a cornerstone to our understand- tilt of the primordial power spectrum is a key parameter ing of cosmology, anchoring theories with precision mea- encoding information about inflation. Beyond the expo- surements of the universe at z=1100, about 380,000 years nentially damped tail, sources of secondary anisotropy after the Big Bang. The CMB alone provides tremendous dominate the power spectrum. The signature imprinted resolving power for the cosmological parameters and it is by inverse-Compton scattering of CMB photons with hot rare to report any parameter measurement without using electrons in the intra-cluster medium, called the Sunyaev CMB data to constrain the fit. Canadian scientists and Zel'dovich (SZ) effect, provides a means of identifying technology have been involved in nearly every important distant galaxy clusters. The surface brightness of the CMB result over the last decade and these measurements SZ effect is insensitive to redshift and closely related to have established the standard cosmological model. cluster mass, making it an ideal selection function for As the Canadian community enters into the process of using the growth of structure through cluster counts to its Long Range Plan, the WMAP collaboration, with in- constrain cosmology. The small scale anisotropies are volvement from CITA and UBC, has published its 7 year affected by weak lensing, providing a measure of the in- results (Jarosik et al. 2010, e.g.) and mapped the CMB tegrated matter along the line of sight which, amongst temperature anisotropies to the cosmic variance limit out other things, can be used to weigh the neutrino species. to ` ∼ 550. By many measures, the WMAP papers are 1.2. CMB Polarization the most cited scientific results in history. The Planck Satellite has been successfully launched and produced its Experiments are only just beginning to have the sensi- First Light survey 6. It is now in a mode of routine ob- tivity necessary to map the faint CMB polarization. The servations. science is tantalizing. While the microwave background With definitive measurements of the temperature provides a data point 380,000 years after the big bang, anisotropy out to moderate angular scales, the CMB gravity waves emitted from an early period of inflation community began refocusing its efforts about 5 years ago carry information from a tiny fraction of a second after on fine angular scale anisotropies (` > 2000) and polar- the big bang. If inflation occurred at a high enough en- ization. In both cases, more capable experiments have ergy scale, as the simplest models suggest, inflationary had to wait for technological breakthroughs{many with gravity waves will imprint a measurable large angular Canadians playing key roles. While early results have scale signature on the CMB polarization. One compo- been reported, definitive measurements will require the nent of this signature, the \B-mode" polarization, pro- better part of the next decade or longer. vides a clean imprint on the microwave background. This is an experimental probe of the highest energy scales and 1.1. Fine Scale CMB Anisotropy earliest time in cosmic history. At small angular scales, secondary scattering with matter along the line of sight provides a weak lensing signa- 1 McGill University ture that is measurable in the B-mode power spectra. 2 University of Toronto 3 Canadian Institute for Theoretical Astrophysics Much of the effort in the international CMB commu- 4 University of British Columbia 5 Author names with (?) have not stated explicitly that they are nity today is focused around galaxy cluster surveys using willing to co-author. fine angular scale CMB anisotropies and making defini- 6 http://www.esa.int/esaCP/SEM5CMFWNZF index 0.html tive measurements of the CMB B-mode power spectrum. 2 2. CANADIAN ACHIEVEMENTS IN CMB RESEARCH OVER THE LAST DECADE Canadian CMB research obtained international recognition with the pioneering measurement (Gush et al. 1990) of the frequency spectrum by the UBC team using COBRA, a sounding rocket experiment. Though results were just slightly behind the COBE satellite spectrum (Mather et al. 1990), the measurements were a factor of several more precise than COBE's initial results. The Gush group's style has been characteristic of the Canadian CMB community: rela- tively small university-based teams involved end-to-end with the experiment, from its initial design conception, its construction, commissioning, deployment, and data analysis. The lion's share of the work has been accom- Fig. 1.| The Atacama Cosmology Telescope (ACT) under con- plished by graduate students and postdocs who receive struction in Port Coquitllam, B.C. April, 2006. ACT has been \old-school" physics training spanning the full life of recording measurements of small angular scale CMB anisotropies for two years and represents the largest number of TES detectors an experiment, something that has become quite rare on the sky in a single experiment. The readout system warm elec- today. The UBC group continued their program by tronics were built at UBC. building and flying BAM (Tucker et al. 1996) aboard a stratospheric balloon to measure the CMB anisotropy. One of the most exciting results in cosmology to date has been the observation of the acoustic peaks in these experiments. The Atacama Cosmology Telescope the CMB power spectrum by the Boomerang collabora- (ACT) (Fowler et al. 2007) currently has the largest tion (de Bernardis et al. 2000; Netterfield et al. 2001), number of bolometers from a single experiment on the providing a measure of the curvature of space-time and sky and has also announced discoveries of new galaxy demonstrating the universe to be flat within a couple clusters using the SZe. A photo of the ACT telescope, percent, as inflation would suggest it should be. Net- under construction in Port Coquitllam, B.C. is shown in terfield was also involved with an earlier observation of Fig. 1. ACT and APEX-SZ are located at 5000 m on the CMB anisotropy (Netterfield et al. 1996) using the SK Atacama Plateau in Chile, very near to the ALMA site. telescope operating from Saskatoon, Saskatchewan. The Canadians play lead roles in the end-to-end realization Toronto experimental cosmology group evolved from this of all three of these experiments (APEX-SZ, SPT, and heritage. ACT): early design, key technology, data analysis, and Not long after the acoustic peaks were observed, the cosmological interpretation. Cosmic Background Imager (CBI) released its results on The Planck Satellite was launched May 14, 2009 and small angular scale anisotropy (Sievers et al. 2002), es- began routine observations from its L2 orbit in July. tablishing the CITA group's expertise in the analysis of Planck has finer angular resolution, more bands extend- large CMB data sets and their cosmological interpreta- ing to higher frequency, higher sensitivity, and better po- tion. larization capabilities than WMAP. It has a good shot at The Wilkinson Microwave Anisotropy Probe (WMAP) an early detection of B-mode polarization from inflation- satellite (Bennett et al. 2003) has revolutionized preci- ary gravity waves and will be a focus for Canadian CMB sion cosmology. Halpern has been involved with the ex- data analysis over the next decade. The Canadian effort periment from the onset and key analyses have taken is supported by Canadian Space Agency (CSA). Dick place at CITA. We note that the Canadian WMAP ef- Bond leads the Canadian team for the High Frequency fort has proceeded without any funding from Canadian Instrument (HFI) and Douglas Scott leads the Canadian sources, other than individual NSERC discovery grants. team for the Low Frequency Instrument (LFI). A recent breakthrough for CMB research was the de- EBEX (Granger et al. 2008) and SPI- velopment of technology that allows the construction DER (Crill et al. 2008) are two new CMB polarization of radiometers with thousands of pixels operating near experiments that will be flown aboard NASA strato- the photon noise limit. The increased sensitivity al- spheric balloons. The McGill group is heavily involved lows for precise measurements on small angular scales with EBEX (Figure 2), while Toronto and UBC play and for observation of the faint polarization signature. leading roles with SPIDER. Both experiments rely The Sunyaev-Zel'dovich Effect (SZe) maps of the Bul- heavily on Canadian technology including the attitude let cluster (Halverson et al. 2009) using the APEX-SZ control systems and the multiplexed readout systems.

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